1. Technical Field of the Invention
The present invention relates to microwave cooking of a food item. More particularly, the present invention relates to microwave food packages which include means for impedance matching microwave energy in a microwave oven to more evenly distribute microwave energy within a food item without interacting with the microwave energy to produce heat.
2. Description of the Prior Art
The popularity of microwave ovens for cooking all or part of a meal has led to the development of a large number of food packages capable of cooking a food item in a microwave oven directly in the food package in which it is stored. The convenience of cooking food in its own package or a component thereof appeals to a large number of consumers. However, one dissatisfaction of microwave cooking for some foods is the inability to heat or warm the center of the food without burning or severely dehydrating the exterior thereof. In particular, larger servings are very difficult to heat uniformly using conventional food packages in a microwave oven. Even when the outer portions are thoroughly cooked, the center is generally undesirably cool.
Microwave interactive films have been produced which are capable of generating heat at the food surface to crispen some food products. U.S. Pat. No. 4,641,005, issued to Seiferth and assigned to James River Corporation of Virginia, assignee of the present application, discloses a microwave interactive material useful in food packaging which is capable of browning the surface of a food item. Specifically, the interactive material includes a very thin metal film applied to a polymer material which is adhered to a rigid substrate. Such a film actually interacts with microwave energy to produce heat at the surface of the food. The heat provided by such an interactive material is advantageous for browning the surface of a food item, but is not advantageous for cooking a thick food item having a large dielectric constant because the outer portion of the food will cook even faster than without interactive material resulting in a deficiently heated inner portion.
Additional microwave heating devices have also been developed primarily for use in food packaging. U.S. Pat. No. 4,876,423, issued to Tighe et al., discloses a medium for producing localized microwave radiation heating wherein the medium is formed from a mixture of polymeric binder and conductive and semiconductive particles that can be coated or printed on a substrate. Again, however, such a medium is designed to interact with the electromagnetic, microwave energy to produce heat and thereby, brown or crispen the surface of a food item, while providing no enhanced heating of the center of the food.
A number of microwave food packages or containers have also been developed which are designed to uniformly heat or adjust the reflectance, transmittance, or absorbance of microwave energy. U.S. Pat. No. 4,266,108 to Anderson et al. discloses a microwave heating device which includes both a microwave reflective member and a microwave absorbing member spaced apart a distance sufficient to provide a temperature self-limiting device. As provided in the above-noted patents, however, the device includes a heater member which interacts with the microwave energy to produce heat and, thus, conductively heats the food item.
Further, U.S. Pat. No. 4,927,991 to Wendt et al. is directed to a food package which discloses a susceptor or heater element in combination with a grid wherein the susceptor surface may be tuned to a matched impedance for maximum microwave power absorbance. Specifically, the reflectance, transmittance and absorbance of the heater can be adjusted by changing certain design factors, including the grid hole size, the susceptor impedance, the grid geometry, the spacing between the grid and the susceptor and the spacing between adjacent holes. The food items contemplated for cooking in such a package is similar to those noted above, particularly food items which require some amount of surface browning or crisping, such as pizza, fish sticks or french fries. Moreover, the problem of adequately heating the center of these types of foods is not required by this device, due to their relatively thin overall nature.
Containers have been also developed which include specially designed covers or lids which are capable of modifying microwave field patterns and which may undergo a change in dielectric constant during microwave heating thereof to alter the heating distribution within the container as heating proceeds. U.S. Pat. No. 4,888,459, issued to Keefer, discloses a microwave container which includes a dielectric structure to provide these properties. Specifically, Keefer discloses a container which may include a lid having a single or a plurality of metal plates or sheets located thereon. A higher electrically thick region may be formed from a dispersion of metal particles in a matrix wherein the dielectric constant of the higher electrical portion is disclosed to be in the range of 25 to 30 for a nonlimiting region. Further, the region may be lossy in character which allows the region, at least initially, to be microwave absorptive, and thus, heat up when exposed to microwave energy. In addition, the region of greater electrical thickness may actually undergo a decrease in dielectric constant during the coarse of microwave heating. Unfortunately, the region or regions of greater electric thickness disclosed by Keefer in this reference and a related U.S. Pat. No. 4,866,234 are at least partially interactive with microwave energy. As a result, the region will produce heat during microwave cooking which may not be desired for certain food items, such as pot pies or fruit pies. Furthermore, without the "shut-off" feature, the production of heat may also create a scorching or fire hazard for food items which require an extended cooking time.
Keefer also discloses in U.S. Pat. No. 4,656,325 a microwave heating package which includes a cover arrangement for use with microwave reflective foodstuff holding pans, such as aluminum foil pans. The cover is compared to a non-reflective coating in optics because it permits microwave radiation into the container holding the foodstuff, while substantially preventing escape of microwave radiation reflected from the foodstuff surface and the container bottom to thereby trap or concentrate the energy within the container. The cover disclosed in the '325 patent is designed to provide, among other things, browning and/or crisping of the surface of the foodstuff.
Food wraps have also been developed for surface heating a food item with variable microwave transmission. U.S. Pat. No. 4,972,058 to Benson et al. discloses a composite material for the generation of heat by absorption of microwave energy comprising a porous dielectric substrate and a coating including a dielectric matrix and flakes of microwave susceptive material. The aspect ratio of the flakes is at least 10. The flake material used in the composite material disclosed by Benson et al. is limited, however, to jagged edged metal flakes.
Consequently, a microwave package is needed which includes a means for uniformly and evenly elevating the temperature of a food item, particularly a food item having a high dielectric constant. Specifically, a microwave package element having a high dielectric constant which does not interact with microwave energy to produce heat and is capable of elevating the temperature of a food item in predetermined areas dependent upon the size and shape of the element is needed for thick food items.